1. Field
The disclosed concept relates generally to security devices having an optically variable device (OVD) portion and, more particularly, to security devices having OVDs that exhibit both Lippmann-Bragg-type optical color change as well as diffractive optical effects. The disclosed concept also relates to methods for creating such security devices.
2. Description of Related Art
An optically variable device (OVD) is a visual device that creates a change or shift in appearance, such as, for example and without limitation, a change in color, when observed from different relative observation points. The evolution of the OVD as a security device stems largely from the search for a mechanism to resist counterfeiting of certain articles and products, or alternatively to render such copying obvious. For example, and without limitation, paper money, banknotes, certificates, security labels, product hang tags, drivers' licenses, ID cards, and credit cards, among other things, frequently employ one or more OVDs to resist counterfeiting or to verify authenticity.
A counterfeiting deterrent employed in some OVDs involves the use of one or more diffractive images that exhibit optical effects which cannot be reproduced using traditional printing and/or photocopying processes. Such images may be, for example, volume holograms or diffractive grating structures (also known as surface relief holograms). When an OVD including such an image is viewed from a predetermined location and tilted so that it is viewed from a different relative location, an optical effect results, such as, for example and without limitation, movement of the image or a change in color. However, additional unique effects are continually needed to stay ahead of the counterfeiters' ability to access or simulate new imaging technologies. Accordingly, other security mechanisms having image-related optical effects have evolved over time.
One such optical effect is color-shifting created by constructing the OVD with a layer or partial layer of a liquid crystal material. The use of films of color-changing liquid crystal materials in OVDs is known in the art. Such films are typically produced by coating liquid crystal material onto a substrate, aligning the molecular structure so that the molecular structure takes on the form of a highly ordered planar Lippmann-Bragg structure, and finally cross-linking the liquid crystal to preserve the aligned planar molecular structure. The Lippmann-Bragg structure has the optical property of reflecting a narrow band of wavelengths of light by the process of refraction from the body of the aligned liquid crystal. Importantly, as the plane of the liquid crystal is tilted relative to the illuminating and viewing angle, the wavelength of the selective band of reflected light shifts to a different color, the magnitude of the shift being dependent on the degree of tilt. This effect is difficult to simulate and can form the basis of a useful security device.
The combination of liquid crystal layers exhibiting a color-changing effect as well as surface relief structures or holograms exhibiting a diffractive optical effect offers an even greater level of security. An OVD exhibiting this combination of effects may be realized either by laminating a liquid crystal film to an embossed surface relief hologram film, or by embossing a surface relief diffractive or holographic structure into one surface of an aligned liquid crystal film. For example, U.S. Pat. No. 6,628,439 to Shiozawa describes authentication films formed by both of these methods. Films exhibiting both color-changing and diffractive effects may be usefully employed in security devices to protect documents or articles of value. The security provided by the diffractive or holographic image is supplemented by the color-changing background effect provided by the liquid crystal layer. However, as such a device essentially comprises two separate optical structures (e.g., the surface relief and the aligned liquid crystal), the security of the device can be compromised if the structures are separated, copied or altered separately and then re-joined together.
There is still a need, therefore, for an OVD that combines the color-shifting and diffractive optical effects, but is not easily separable into two component parts which allow the two optical effects to be copied or altered separately.